****su,
Old home copy of AIP, but this stuff doesn't change too often so should be correct. ENR 1.6.8.1.8
When operating in, or in the vicinity of, a GAAP control zone, a transpoonder should be selected to:
a) STANDBY - for flights wholly within a GAAP CTR
b) ON/ALT - prior to take-off departing a GAAP CTR, or
c) ON/ALT - when operating in GAAP lanes of entry.
I suspect it's because GAAP's are busy (well, used to be anyway) with vfr traffic and it removes the clusters of black tracks and labels from the screens.